Position a rack in the lower third of the oven and preheat the oven to 325∞F. Line the bottom and sides of the baking pan with parchment paper or foil, leaving an overhang on two opposite sides.

For Brownies:

Combine the butter, sugar, cocoa, and salt in a medium heatproof bowl and set the bowl in a wide skillet of barely simmering water. Stir from time to time until the butter is melted and the mixture is smooth and hot enough that you want to remove your finger fairly quickly after dipping it in to test. Remove the bowl from the skillet and set aside briefly until the mixture is only warm, not hot.

Stir in the vanilla with a wooden spoon. Add the eggs one at a time, stirring vigorously after each one. When the batter looks thick, shiny, and well-blended, add the flour and stir until you cannot see it any longer, then beat vigorously for 40 strokes with the wooden spoon or a rubber spatula. Stir in the nuts, if using. Spread evenly in the lined pan.

For Cream Cheese Batter:

Beat cream cheese with granulated sugar until light and creamy.Add in the egg yolk and beat until well mixed. Scoop out about 1/2 cup of cream cheese into a small bowl and stir in matcha powder. Pour the matcha cream cheese mixture into a small ziplock bag. Add the lemon zest into the remaining cream cheese, and pour into another small ziplock bag.

Using a scissor, cut a small hole on the edge of the zip lock bags and pipe out whatever pattern that strike your fancy on top of the brownie dough.

Bake, until a toothpick plunged into the center, emerges slightly moist with batter, 20 to 25 minutes. Let cool completely on a rack.

Lift up the ends of the parchment or foil liner, and transfer the brownies to a cutting board. Cut into 16 or 25 squares.

Chocolate note: Any unsweetened natural or Dutch-process cocoa powder works well here. Natural cocoa produces brownies with more flavor complexity and lots of tarts, fruity notes. I think it’s more exciting. Dutch-process cocoa results in a darker brownie with a mellower, old-fashioned chocolate pudding flavor, pleasantly reminiscent of childhood.